Coal is a brown to black combustible rock which is used for a variety of applications, e.g., generating electrical power, in metallurgical production, in general industrial processes and in residential-commercial uses. Generally, coal is burned in fire-tube or water-tube boilers for generating steam that is, in turn, used to produce electricity, provide heat for factories and buildings and to provide steam for production processes.
Coal is mined in various geographical regions, including many regions of the United States. The composition of coal may vary considerably depending on the geographical region from which it is mined, and further may vary significantly within the same geographical region.
Depending on the particular region from which coal is obtained, coal may comprise various elements that, upon combustion, are emitted into the surrounding atmosphere as pollutant gases. The nature and amount of these gases will vary from one type of coal to another. For example, while coal from the Illinois basin releases a substantial amount of sulphur dioxide gas upon combustion, combustion of Wyoming coal yields a comparatively small amount of sulphur dioxide gas.
In view of increasing interest in protecting the environment from harmful pollutants, governmental regulations have been enacted to limit the amount of pollutants that may be released into the atmosphere upon combustion of coal. One such pollutant is sulphur dioxide gas. Sulphur is present in coal in two principle forms. These two forms are organic sulphur and inorganic sulphur compounds, e.g. iron pyrite. Upon combustion of coal that includes inorganic and organic sulphur compounds, sulphur dioxide emissions are released into the surrounding atmosphere. In order to comply with governmental regulations that limit the amount of sulphur dioxide emissions, coal that comprises high levels of sulphur must be treated in a manner that reduces the sulphur dioxide emissions to a level below that set by the governmental regulations. One treatment process that has found applicability in reducing the sulphur dioxide emissions following combustion of high sulphur containing coal is to coat the coal with calcium to form "clean coal." Upon combustion of calcium coated coal, the organic sulphur present in the coal reacts with the calcium to form calcium sulfate instead of sulphur dioxide. The resultant calcium sulfate remains in the coal ash with a concomitant reduction in sulphur dioxide emissions.
Another element present in some coals that gives rise to pollutant emissions upon combustion is chlorine. When chlorine is present in coal, it is present in three forms: as chloride ions dissolved in moisture present in the coal, chloride ions precipitated on the coal surface and as "organic chlorine." Organic chlorine is theorized to actually be chloride ions paired with positive groups on the coal surface. Upon combustion of coal comprising organic chlorine, hydrogen chloride gas is emitted into the atmosphere. This hydrogen chloride gas has been determined to be a pollutant, similar to sulphur dioxide gas. Governmental regulations are scheduled to take effect that will limit hydrogen chloride gas emissions upon combustion of coal comprising organic chlorine.
Although environmental regulations that limit the amount of pollutant gas emissions upon combustion of coal are effective in preserving the environment, they also make certain coal reserves in the United States unprofitable to exploit. For example, the coal reserve of the Illinois basin, which is the second largest coal reserve in the United States, comprises coal with comparatively high levels of both sulphur and chlorine. Thus, combustion of Illinois basin coal yields substantial emissions of both sulphur dioxide and hydrogen chloride gases. Because treatment processes for limiting the emissions of sulphur dioxide and hydrogen chloride gases are expensive, exploitation of Illinois basin coal has become increasingly unprofitable. It is often cheaper for many utilities, as well as other coal using groups, to comply with the government environmental regulations that limit emissions of these gases by switching to coals that include comparatively lower concentrations of sulphur and chlorine, such as Wyoming coal.
Thus, there is substantial interest in finding an economical treatment for coal that includes high concentrations of sulphur and chlorine which would reduce the resultant levels of both hydrogen chloride and sulphur dioxide emissions upon combustion. The ideal treatment would return those coal reserves rendered unprofitable through governmental regulation to profitability by a simple and economical procedure.